The present invention relates to electrical devices employing partially pyrolyzed polymer filaments and methods for making such devices.
Pyrolyzed carbonaceous fibers have been the subject of experimental investigation for a number of years.
It is known in the prior art to pyrolyze polymers at a range of temperatures. Several U.S. patents disclose pyrolyzation at temperatures as low as 500.degree. C. in processes for improving the physical properties of the raw fiber, such as tenacity or Young's modulus. See U.S. Pat. Nos. 3,673,035 to Whitney and 4,069,297 to Saito and British Pat. Nos. 1,257,481 to Rolls-Royce and 1,344,374 to Sosedov et al. Other patents teach carbonization temperatures on the order of 700.degree. C. See U.S. Pat. Nos. 3,285,696 to Tsunoda, 3,497,318 to Noss, 3,533,743 to Prescott, 3,607,059 to Joo, 3,988,426 to Ogawa et al, 4,237,108 to Fukuhara et al and 4,237,109 to Hiramatsu et al, and British Pat. No. 1,241,937 to Monsanto.
The conductive properties of carbon filaments have been exploited, for example, in making conductive moldable materials as discussed in U.S. Pat. No. 3,406,126 to Litant. It has also been proposed to employ low resistivity, pyrolyzed carbon fibers as light weight electrical conductors. Accordingly, changes in resistivity with pyrolyzation temperature have been the subject of experimentation.
The electrical conductivity of oxidized polyacrylonitrile (PAN) fiber has been studied as a function of heat treatment temperature between 710.degree. K. and 950.degree. K. See N. R. Lerner, "Electrical Conductivity and Electron-Spin Resonance In Oxidatively Stabilized Polyacrylonitrile Subjected to Elevated Temperature", J. Appl. Phys. 52 (11), November 1981. The article indicates that resistivity measurements of pyrolyzed fiber were made after the resistance reading was constant for at least 1 minute. While Lerner reports variations in resistivity with pyrolyzation temperature, no non-ohmic effects are noted.
Brom et al. have studied the conductivity of pyrolyzed polyimide (KAPTON) film as a function of pyrolysis temperature. Brom et al, "On New Conducting Polymer-Pyrolyzed Kapton", Solid State Communications, Vol. 35, p. 135 (Pergamon, 1980). Brom et al cut the pyrolyzed film into rectangular or needle shapes to measure the electronic properties of the material. At a controlled measurement temperature of 4.2.degree. K., Brom et al. report that no deviation from ohmic behavior was seen up to voltage gradient of 2.times.10.sup.3 v/cm.
Gittleman et al, postulate a structure for pyrolyzed polyimides in their article, "Are Pyrolyzed Polyimides Conducting Polmers?", Journal of Electronic Materials, Vol. 10, No. 2 (1981). Gittleman et al also suggest the application of higher fields to pyrolyzed polyimide film samples to test the validity of a theoretical "charging energy" model.
Electrical switching properties have been observed in evaporated carbon and glassy carbon coatings, heat treated to a maximum temperature of 600.degree. C. See Antonowicz et al, "Switching Phenomena in Glassy Carbon," Carbon, 1973 Vol. 14, pp. 1-5. In the Antonowicz experiments, a quartz plate with one aluminum electrode was covered with polyfurfuryl alcohol solution and the plate was heated. Once carbonization was accomplished, an upper aluminum electrode was deposited by evaporation to form a sandwich. The samples exhibited switching behavior, but remained switched for long periods of time, for example, one to three days. The experimental samples were apparently made as a part of a study of fundamental physical properties of carbon, and the article does not teach the use of the samples as electronic devices. Antonowicz et al conclude that the samples are "very difficult" to prepare with a "sufficient degree of reproducibility for basic research."
It is a basic object of the present invention to provide a non-ohmic fiber element useful in the fabrication of electronic devices.
It is another object of the present invention to provide useful electronic devices made from partially pyrolyzed polymer fibers, and to provide methods of making such devices. These devices include: electronic switches, responsive to electrical stimulae; bipolar, high current switches and varistors; and environmentally responsive switches such as thermal switches; and memory elements switchable between a high and a low resistance state.
These and other objects and features will be apparent from the following description and claims, when read with the accompanying drawings.